Among the early impairments typically shown by children with autism is a failure to imitate the motor actions of other people. Imitation plays a central role in both in both social and cognitive development. Thus, autistic children's deficient imitation ability may help to explain many of the core symptoms of autism. Although impaired motor imitation is considered a diagnostic criterion for autism, very little is known about the nature or cause of this impairment. The primary aim of this research project is to explore the psychological nature and neurophysiological basis of autistic children's deficit in motor imitation. Three possible reasons for autistic children's impairment in motor imitation will be investigated using rigorous experimental methods that have been designed to assess imitation in normally-developing infants. Subjects will consist of developmentally-matched samples of young autistic, mentally-retarded, and normally-developing children. The first experiment is designed to determine whether the autistic child has difficulty recognizing the similarity between actions of the self and other, a possibility that has been raised by investigators in the field but has never been directly tested. The second experiment is designed to assess whether the autistic child, although recognizing the similarity between actions of self and other, is unable to translate this perceptual information into motor actions (immediate imitation). The third experiment is designed to assess whether the autistic child's failure to imitate is related to a general deficit in the ability to store and later act on internal representations of absent events (deferred imitation). Such a representational capability has been linked to the prefrontal cortex. To further explore the relations between frontal lobe functioning and motor imitation in autism, both behavioral and electrophysiological (EEG) measures of frontal lobe functioning will be obtained to examine whether these measures are predictive of children's level of imitation and social ability.